1. Field of the Invention
The object of the present invention is a method for manufacturing surgical implants to be used in particular but not exclusively, for manufacturing internal articular prostheses. It also concerns an implant constructed according to said method.
2. Description of the Prior Art
It is generally known that the construction of internal prostheses gives rise to numerous problems, in particular a problem of biocompatibility with the surrounding living tissues and problems connected with the chemical and/or mechanical interactions of the prosthesis with said tissues or even between two distinct parts of the prosthesis.
These problems arise in a particular keen way for articular prostheses which are often subjected to considerable mechanical constraints while having to carry out a complex articulation function.
Thus, for this type of prostheses, there are further problems relating to the surfaces of contact of the different parts of the articulation which undergo relative displacements and are thus subjected to frictions.
It is clear that these contact surfaces, by which the articulation function is re-established and on which the transmission of forces takes place must have a very high precision, be smooth and hard, and have a very low friction coefficient, and more particularly a very low index of roughness.
This is why numerous researches have been carried out up to now for finding a material having all the properties required for resolving the above mentioned problems. Thus, in the field of highly resistant materials, metals or metal compounds have been proposed such as cobalt, chromium, molybdenum alloys or stainless steel. However, it has proved that these materials have a certain number of disadvantages, more particularly in so far as their weight is concerned (density which is much greater than that of bone (8 to 10 times greater)) and shaping thereof. Another disadvantage of stainless steels comes from the fact that in them the chromium which is present in these stainless steels and which serves as protection agent against corrosion disappears in the presence of chlorine ions therefore, the prosthesis which is subjected to the action of the chlorine ions present in the surrounding living tissues, will be no longer protected against corrosion and will finish by breaking whatever its weight and its volume.
Implants made of carbon/resin composites have been also tested. However, these composites have been rejected becaue of the toxicity of the resin used up to now.
Because of this toxicity researches have then turned towards the production of a carbon-carbon composite whose advantages may be briefly resumed as:
a priori perfect biocompatibility, PA1 density very close to that of the cortical bone, PA1 the possibility of obtaining a heterogeneous structure, PA1 elasticity very close also that of the boney cortical substance, PA1 biochemical and electrochemical inertia. PA1 the formation of an implant blank in a metal or composite substrate comprising at least one contact bearing surface substantially to the dimensional tolerances of those of the finished product; PA1 a first polishing of this contact surface; PA1 preparation of the surface of this contact portion once polished, this preparation consisting in cleaning with physico-chemical means; PA1 a first treatment phase comprising decontamination of this surface by bombardment with high energy ions coming from an evaporation source, the first treatment phase being carried out in a reactor having a structure similar to that of a thermochemical treatment oven with ionic bombardment in a treatment atmosphere comprising a rare gas such as argon or nitrogen at a pressure of 10.sup.-6 T to a few millitorrs, the pieces subjected to this treatment being brought to a cathode potential higher than 800 volts so as to repulverise the incident ions; PA1 heating of the implant to a predetermined temperature compatible with that at which said deposition is to be carried out, this heating being at least partially provided solely by transformation of the kinetic energy of the ions into heat energy; PA1 once said temperature has been reached, the formation of said deposition by creation of metal vapor inside the reactor, by introducing into the reactor a reactive gas and by reducing the cathode potential of the implant to a value which may be between 100 and 400 volts; PA1 final polishing of the surface of the bearing portion comprising said deposition. PA1 a first type in which the metal vapor is obtained by thermal evaporation for example by Joule effect, by induction, by laser radiation, by means of an electron beam gun, or a hollow cathode gun or even using electric arcs; PA1 a second type in which the metal vapor is obtained by cathode spraying, in a diode, triode, magnetron or ion beam gun type circuit.
However, it has proved that this material does not provide contact surfaces having the required properties, more particularly in so far as the hardness and resistance to abrasion are concerned.
This is why attempts have been made to carry out treatments on these contact bearing surfaces for obtaining sufficiently smooth and hard surface conditions. However, the results of friction tests carried out on contact bearing surfaces treated with conventional methods, more particularly by deposition of titanium nitride according to the methods of the CVD ("Chemical Vapor Deposition") type have not been thought sufficient and the use of this composite material has been temporarily rejected and similar tests have been carried out on implants made from titanium alloy, in particular TA 6 V which comprises 90% titanium, 6% aluminium and 4% vanadium.
In this case also, the problem of treating the contact bearing surfaces is just as keen with in addition the problems relative to the deformations undergone by the metal at the treatment temperatures (usually high), these deformations of course leading to inadmissable defects of precision of the implant, once treated.
The invention overcomes these problems.